1. Field of the Invention
The invention relates to a shift control apparatus of a continuously variable transmission (CVT) that continuously (i.e., steplessly) changes speed ratios. More particularly, the invention relates to a shift control apparatus that determines whether that shift control apparatus is operating normally, as well as to a method of determining whether a shift control apparatus is operating normally.
2. Description of the Related Art
A vehicle is known which has a continuously variable transmission that continuously changes speed ratios provided in a power transmitting path between an engine and driving wheels. In this continuously variable transmission, the actual speed ratio calculated based on the input shaft rotational speed and the output shaft rotational speed of the continuously variable transmission is normally shift-controlled to a target speed ratio determined based on the vehicle speed and accelerator operation amount, for example. Shifting in the continuously variable transmission is normally performed by a hydraulic control circuit in the continuously variable transmission. However, if a solenoid valve or the like arranged in the hydraulic control circuit fails, shift control is unable to be performed normally. Therefore, it is desirable to determine whether shift control will be performed normally.
Thus, Japanese Patent Application Publication No. 8-233083 (JP-A-8-233083) describes technology which calculates an absolute value of the difference between a target speed ratio of a continuously variable transmission (a toroidal-type continuously variable transmission) and the actual speed ratio of the continuously variable transmission, and determines that the continuously variable transmission is operating normally if the change in the absolute value of that difference is small. Also, Japanese Patent Application Publication No. 8-127261 (JP-A-8-127261) describes technology that determines whether shift control is normal based on whether a difference between a target speed ratio and an actual speed ratio continues to exceed a set value for a predetermined period of time.
In addition to a shift control solenoid valve, a plurality of other valves, such as linear solenoid valves which control the line pressure, i.e., the base pressure, are also provided in the hydraulic control circuit in JP-A-8-233038 and JP-A-8-127261 and the like. For example, if a linear solenoid valve that controls the line pressure fails, sufficient line pressure will no longer be able to be supplied to the various valves in the hydraulic control circuit. As a result, the difference between the actual speed ratio and the target speed ratio will increase so it will be determined that shift control will not be able to be performed normally. That is, even if the shift control solenoid valve is operating normally, it will be determined that an abnormally has occurred in the shift control apparatus. In this way, because it will be determined that shift control is abnormal due to the failure of another part, even if the shift control solenoid valve is operating normally, it is not possible to accurately determine whether the shift control solenoid valve is operating normally by only the determination based on that difference.